RFR(M): 8073315: Enable gcc -Wtype-limits and fix upcoming issues.

[Lindenmaier, Goetz]

Hi,

We fixed all the warnings showing with -Wtype-limits and gcc in our VM.
I would like to contribute these small changes.

This warning detects unnecessary compares for >= 0 of unsigned values.

At least one warning unveils a real bug. In memory/blockOffsetTable.cpp,
BlockOffsetArrayContigSpace::last_active_index(), the compare to >=0
tries to prevent an underflow, but the value is unsigned.

Further useless compares in real code are in os_linux.cpp,
opto/graphKit.cpp, prims/jvmtiRedefineClasses.cpp

In some places debug code with constant debug flags caused
warnings in the opt build (g1/concurrentMark.cpp, classfile/systemDictionary.cpp)

The remaining warnings occur in asserts, which is also the biggest part.

The change also removes an empty #ifdef ASSERT .

Please review this change.  I please need a sponsor.
https://bugs.openjdk.java.net/browse/JDK-8073315
http://cr.openjdk.java.net/~goetz/webrevs/8073315-wUnsign/webrev.01/

I tested this on linuxx86_64 with gcc 4.1.2, 4.3 and 4.8.  Further I ran the patch
with our nightly builds and tests of jdk9,see
http://cr.openjdk.java.net/~simonis/ppc-aix-port/index.html

Best regards,
  Goetz.